Separation of metal-organic frameworks

ABSTRACT

A method of separating a metal organic framework (MOF) from a solution and associated apparatus. The method comprises: providing a MOF containing solution; contacting the MOF containing solution with an acoustic reflector surface such that, any high frequency ultrasound applied within the MOF containing solution reflects off the acoustic reflector surface; and applying a high frequency ultrasound of at least 20 kHz to the MOF containing solution. The MOF material is substantially separated from solution as aggregated sediment that settles out of solution.

CROSS-REFERENCE

The present application is a Continuation of U.S. patent applicationSer. No. 15/577,124 filed 27 Nov. 2017, which is a U.S. National StageApplication under 35 U.S.C. § 371 of International Patent ApplicationNo. PCT/AU2016/050410 filed 26 May 2016, which claims the benefit ofpriority to Australian Provisional Patent Application No. 2015901950filed 27 May 2015, the disclosures of all of which are herebyincorporated by reference in their entireties. The InternationalApplication was published in English on 1 Dec. 2016 as WO 2016/187669.

TECHNICAL FIELD

The present invention generally relates to a method, system andapparatus for the separation for metal-organic frameworks (MOFs). Theinvention is particularly applicable for separation of MOFs from a MOFcontaining solution, as well as the separation of contaminants presentin and around a MOF from the MOF and it will be convenient tohereinafter disclose the invention in relation to those exemplaryapplications. However, it is to be appreciated that the invention is notlimited to that application and could be used in any MOF separation orpurification step, process, system and/or apparatus.

BACKGROUND OF THE INVENTION

The following discussion of the background to the invention is intendedto facilitate an understanding of the invention. However, it should beappreciated that the discussion is not an acknowledgement or admissionthat any of the material referred to was published, known or part of thecommon general knowledge as at the priority date of the application.

Metal-Organic Frameworks (MOFs) are a class of promising porousmaterials having tuneable functionality, large pore sizes and thehighest known surface areas. These characteristics are of high interestfor a myriad of industrial applications such as gas storage, gasseparation, drug delivery and catalysis. However, to date the cost ofthese materials has remained prohibitively high, thereby restricting theability of these materials to make a significant impact on prospectivemarkets or technologies. Only seven MOFs out of the thousands of MOFSdescribed in academic literature are commercially available, with thatavailability limited to small quantities (grams).

An important requirement for accessing the potential applications ofMOFs is the ability to routinely synthesise MOF materials in largequantities (kg scale or higher) at an economic price point. Such aprocess needs to be a versatile, efficient and scalable synthesis thatis able to produce MOFs in large quantities in order to introduce thesematerials to real world applications.

However, traditional laboratory routes such as the classicalsolvothermal synthesis are difficult to scale up due to the extendedreaction times (˜24 hours) and low quality material yield. Furthermore,a wide variety of available synthetic synthesis methods have a singularnature providing an inherent inflexibility for any prospectiveproduction process.

Large-scale process post synthesis steps such as cleaning, separationand activation can also be crucial for cost-effective production of highquality MOF material.

There are a number of known technologies for solid-liquid separationincluding centrifuges, cyclones, electrostatic precipitators, settlingchambers, classifiers or filters, and evaporation. However, the smallsize of the MOF particles, their low concentration in the solvent, aswell as their density approaching that of the solvent (due to the highporosity), makes separation unfeasible, inefficient or expensive at anindustrial scale using most conventional methods.

Sedimentation- or tubular-type centrifuges are known to process solidsas small as 100 nm, while wet electrostatic precipitators can alsohandle particles above 50 nm with 98% efficiency and greater than 100 nmwith 95% efficiency. Yet, a high capital investment is generallyrequired and this type of equipment also has generally high overallpower consumption.

Liquid-solid filters can process particles as small as 500 nm, whenoperated under pressure (typically 2-15 bar for continuous rotary drum,or 3 to 70 bar for batch), or when using cartridge or sand (packed bed)filters (batch operation). However, filters requiring a high pressuredrop across the separation stage are subject to fouling over time whilepressurised rotary drum filters have a very high capital cost.

It would therefore be desirable to provide a new and/or improved methodof separating and/or purifying a MOF in solution from contaminants,without destroying the integrity of the porous MOF.

SUMMARY OF THE INVENTION

The present invention provides a new separation apparatus, system andmethod that can separate and/or purify a metal organic framework contentin a solution. The present invention also relates to a method, systemand apparatus for washing and/or purification of metal-organicframeworks to remove contaminants, such as occluded unreacted ligands,from within pores of the metal organic framework in solution.

In a first aspect, the present invention provides a method of separatinga metal organic framework (MOF) from a solution, comprising:

providing a MOF containing solution;

contacting the MOF containing solution with an acoustic reflectorsurface such that, any high frequency ultrasound applied within the MOFcontaining solution reflects off the acoustic reflector surface suchthat a standing wave is formed through constructive interference; and

applying a high frequency ultrasound of at least 20 kHz, preferablybetween 20 kHz to 4 MHz, more preferably 500 kHz to 2 MHz, yet morepreferably between 800 kHz and 2 MHz, and yet more preferably between 1MHz and 2 MHz to the MOF containing solution;

thereby substantially separating the MOF from solution as an aggregatedsediment which settles out of solution.

In some embodiments, a high frequency ultrasound of at least 400 kHz isused.

The present invention provides an ultrasonic and/or megasonic separationprocess, system and apparatus involving the application of highfrequency ultrasound or megasonic frequencies to a MOF containingsolution. Acoustic radiation from the applied ultrasonic (and in someembodiments megasonic) frequencies aggregate MOFs towards pressure nodesformed within the MOF-containing solution. The aggregated MOF materialtends to sediment out of solution at a greatly accelerated rate to thebottom of a container or separation chamber housing the MOF containingsolution. Ultrasonic and megasonic operation involves no moving parts,has a low surface area of contact with the fluid (i.e. lower capacityfor fouling, ease of cleaning) and allows continuous separation, washingand/or purification of MOFs. Furthermore, the simplicity and speed ofthe process enables the process to be scaled, and applied economicallyto an existing MOF production method.

It should be appreciated that the separation or purification of the MOFaccording to the present invention occurs post-synthesis of the MOF incrude or pure form. Accordingly, the method of the present invention isused to separate MOF as an aggregated sediment in solid form, from a MOFcontaining solution once MOF synthesis is completed, or from a MOFcontaining solution in which the MOF has been added to that solution fora purpose or function or similar post MOF synthesis application.

It also should be appreciated that the MOF containing solution typicallycomprises a mixture or suspension of MOFs within the solution. In thisregard, the MOF comprises a solid component or particles which aredispersed throughout the liquid of that solution. For ease of reference,this suspension of MOFs in solution will be referred to as a “MOFcontaining solution” in this specification. Typically, the MOFcontaining solution comprises MOFs which have unreacted ligands, metalsalts or other contaminants trapped within the pores of the MOF.

The step of contacting the MOF containing solution with an acousticreflector surface can comprise any arrangement in which the MOFcontaining solution is contacted or otherwise has a fluid connectionwith the reflector surface. In some embodiments, the step of contactingthe MOF containing solution with an acoustic reflector surface comprisespositioning or otherwise providing the acoustic reflector surface withinthe MOF containing solution. The acoustic reflector surface ispreferably spaced away from the source of the high frequency ultrasoundapplied within the MOF containing solution that it reflects off theacoustic reflector surface such that a standing wave is formed throughconstructive interference. In this respect, the reflected sound wavesare able to interact with the original transmitted wave. If thereflected and the transmitted wave are in phase, i.e. the peaks andtroughs of the waves are aligned, then constructive interference willoccur leading to resonance. With this occurrence, pressure nodes andanti-nodes will form along the path of the interacting sound waves atdistances equal to multiples of half the wavelength of the waves.

The high frequency ultrasound can be applied or produced from anysuitable device. In embodiments, the high frequency ultrasound isapplied by high frequency transducer. The high frequency transducer andreflector surfaces are preferably parallel spaced apart within orrelative to the MOF containing solution. Again, that spacing is suitablefor the formation of a standing wave through constructive interferenceof the reflected and the transmitted wave (i.e. the reflected and thetransmitted wave being in phase).

In some embodiments, the present invention provides a method ofseparating a metal organic framework (MOF) from a solution, comprising:

providing a MOF containing solution in a housing containing a highfrequency transducer and an acoustic reflector surface, the transducerand the acoustic reflector surface being spaced apart within the housingsuch that a standing wave is formed through constructive interference;and

operating the transducer to apply a high frequency ultrasound of atleast 20 kHz to the MOF containing solution;

thereby substantially separating the MOF from solution as an aggregatedsediment which settles out of the MOF containing solution, leaving anytrapped ligands, metal salts or contaminants in solution.

The use of an acoustic reflector surface assists in the formation of astanding wave field required to form pressure nodes where particles arecollected for cleaning or separation. Standing wave ultrasound fieldsare created by placing a reflecting surface in front of the transducer.The use of an acoustic reflector surface facilitates the formation ofpressure nodes in the present invention.

The frequency of the applied high frequency ultrasound is important inthe function and effect of the separation. Whilst the preferredfrequency depends on factors such as MOF composition, particle size,solution composition and the like, the general ranges of applied highfrequency ultrasound are as follows: In some embodiments, the appliedhigh frequency ultrasound is between 20 kHz to 4 MHz, preferably 500 kHzto 2 MHz, more preferably between 800 kHz and 2 MHz, and yet morepreferably between 1 MHz and 2 MHz. In some embodiments, the appliedhigh frequency ultrasound is greater than 1 MHz, preferably between 1MHz and 10 MHz, and more preferably between 1 and 4 MHz.

The formation of a standing wave depends on a variety of factors,including frequency, transducer and reflector spacing and the like. Forexample, in embodiments it would be possible that a standing wave fieldis formed with 100 kHz in larger vessels with node to node distance of7.2 mm.

In some cases, it can be advantageous to move the applied high frequencyultrasound between a high frequency and a low frequency. In someembodiments, the applied high frequency ultrasound is cycled between ahigh frequency and a low frequency. Again, the selected frequenciesdepend on a number of factors. However, in some embodiments the highfrequency is between 400 kHz to 2 MHz and the low frequency is between20 kHz to 400 kHz. However, other embodiments the low frequency isbetween 20 kHz to 500 kHz and the high frequency is between 500 kHz to 2MHz.

The energy density of the applied high frequency ultrasound is anotherfactor which can affect separation. In some embodiments, the energydensity of the applied high frequency ultrasound is at least 25 kJ/kg,preferably between 100 kJ/kg to 250 kJ/kg.

In some embodiments, the process and apparatus of the present inventionhas the ability to achieve specificity of separation based on particlesize by tuning an operation parameter such as frequency and energydensity. Thus, in some embodiments at least one of frequency or energydensity of the applied high frequency ultrasound is tuned to selectivelyseparate MOF and any contaminants in the MOF containing solution basedon a specific particle size.

MOF material is extremely porous and therefore contaminant species in asolution during synthesis of a MOF can be trapped or otherwise locatedin the pores of the MOF material. Alternatively, the pores within a MOFcould be contaminated or blocked during industrial application of MOFs.The process of the present invention can be used for separation and/orpurification of MOFs from such contaminants, and more particularlycontaminants in the pores of a MOF. Thus, in some embodiments, the MOFpresent in the MOF containing solution metal organic framework (MOF)includes at least one contaminant, and the method substantiallyseparates the contaminant from the MOF within the solution. Thecontaminant is preferably left in solution and the MOF settles at orproximate the bottom of the solution as a solid sediment after treatmentwith high frequency ultrasound. Again, this separation process includesseparation of contaminants in the pores of the MOF. The separationmethod of the present invention therefore provides an advantageousmethod of purifying a crude or contaminated MOF. The method according tothe invention results in separation or purification of MOFs so that itis substantially free from unreacted metal salts, ligands or othercontaminants within the pores of the MOF.

It should be appreciated that separation in this washing and purifyingcontext broadly encompasses a number of unit processes including washingprocesses, purification processes, polishing processes and the like. Allof these processes involve the separation of a product (in the presentinvention a MOF) from a contaminant or other material. It should beappreciated that all these process functions and similar processes areincorporated into the scope of the present invention.

The separation process of the present invention can comprise a washingprocedure or process. In such embodiments, the step of providing the MOFcontaining solution preferably comprises adding a MOF to a washingsolution. The washing solution can comprise any suitable solvent ordispersant. Suitable washing solutions include water, ethanol,dimethylformamide (DMF), methanol, tetrahydrofuran, chloroform, acetone,dichloromethane, ethyl acetate, diethylformamide or a combinationthereof.

The MOF is preferably separated in greater purity from the washingsolution following sedimentation at the bottom of the solution. Theprocess therefore further comprises the step of isolating the MOF (fromthe solution which includes the MOF as an aggregated sediment).Isolation of the MOF can be achieved using any number of separationprocess steps including but not limited to decanting, filtration,evaporation, or centrifugation.

Following isolation, it may be preferable to wash or further treat theresultant MOF. The process of the present invention may thereforefurther comprise at least one additional washing step including thesteps of:

isolating the MOF;

adding the isolated MOF to a washing solution;

contacting the MOF containing washing solution with an acousticreflector surface such that, any high frequency ultrasound appliedwithin the MOF containing washing solution reflects off the acousticreflector surface such that a standing wave is formed throughconstructive interference; and

applying a high frequency ultrasound of at least 20 kHz, preferably atleast 400 kHz, preferably between 20 kHz to 4 MHz, preferably 500 kHz to2 MHz, more preferably between 800 kHz and 2 MHz, and yet morepreferably between 1 MHz and 2 MHz to a MOF containing solution, therebyseparating the MOF, leaving any contaminants in the washing solution.

Again, the frequency of the applied high frequency ultrasound isimportant in the function and effect of the separation. Whilst thepreferred frequency depends on factors such as MOF composition, particlesize, solution composition and the like, the general ranges of appliedhigh frequency ultrasound are as follows: In some embodiments, theapplied high frequency ultrasound is between 20 kHz to 4 MHz, preferably500 kHz to 2 MHz, more preferably between 800 kHz and 2 MHz, and yetmore preferably between 1 MHz and 2 MHz.

In some embodiments, a plurality of washing steps outlined above isused.

In some embodiments, the present invention can be used to separate smallMOF particles from the mother liquid produced after production of a MOF.In such embodiments, the MOF containing solution comprises a motherliquid from a MOF forming process. Furthermore, at least one contaminantcan include occluded unreacted ligands within pores of the MOF.

The present invention can also be used to improve the surface area ofthe final product, providing an advantageous alternative to timeconsuming and costly calcinations traditionally used for surface areaimprovement of MOFs. The process can therefore assist in maintaining MOFproduct quality i.e. porosity, thermal and chemical stability. Thus, insome embodiment the method also improves the BET surface area of theMOF, preferably by at least 20%, preferably 30% compared to a centrifugewashed MOF.

The MOF containing solution comprises a MOF material in a solution. Thesolution can comprise any suitable solvent or dispersant includingwater, ethanol, dimethylformamide (DMF), methanol, acetone,tetrahydrofuran, chloroform, dichloromethane, ethyl acetate,diethylformamide or a combination thereof. Preferably, the MOFcontaining solution is used at ambient or room temperature.

A large variety of MOFs or MOF materials can be used with the presentinvention.

It should be appreciated that Metal Organic Frameworks (MOFs) (alsoknown as coordination polymers) or MOFs are class of hybrid crystalmaterials where metal ions or small inorganic nano-clusters are linkedinto one-, two- or three-dimensional networks by multi-functionalorganic linkers. In this sense, a MOF is a coordination network withorganic ligands containing potential voids or pores. A coordinationnetwork is a coordination compound extending, through repeatingcoordination entities, in one dimension, but with cross-links betweentwo or more individual chains, loops, or spiro-links, or a coordinationcompound extending through repeating coordination entities in two orthree dimensions and finally a coordination polymer is a coordinationcompound with repeating coordination entities extending in one, two, orthree dimensions.

MOFs have many appealing features having surface areas of thousands ofsquare meters per gram, extremely low density, interconnected cavitiesand very narrow porosity distributions. A variety of open micro- andmesoporous structures can be developed, leading to materials withextreme surface area.

Examples of metal organic frameworks which may be suitable for use inthe present invention include those commonly known in the art asMOF-177, MOF-5, IRMOF-1, IRMOF-8, Al-fum (Aluminium fumarate), Zr-fum(Zirconium fumarate), UiO-66, HKUST-1, NOTT-400, MOF-74 and MIL-53(aluminium terephthalate). It should be appreciated that the presentinvention is suitable for use with a large number of MOFs and shouldtherefore not be limited to the exemplified MOF structures in thepresent application.

MOFs used in the process of the present invention preferably comprise aplurality of metal clusters, each metal cluster including one or moremetal ions; and a plurality of charged multidentate linking ligandsconnecting adjacent metal clusters. Such MOFs can therefore be moregenerally defined by the charged multidentate linking ligands connectingadjacent metal clusters which are used to form each MOF.

Each metal cluster preferably includes one or more metal ions. As usedherein, the term “cluster” means a moiety containing one or more atomsor ions of one or more metals or metalloids. This definition embracessingle atoms or ions and groups of atoms or ions that optionally includeligands or covalently bonded groups. Each cluster preferably comprisestwo or more metal or metalloid ions (hereinafter jointly referred to as“metal ions”) and each ligand of the plurality of multidentate ligandincludes two or more carboxylates.

Typically, the metal ion is selected from the group consisting of Group1 through 16 metals of the IUPAC Periodic Table of the Elementsincluding actinides, and lanthanides, and combinations thereof.Preferably, the metal ion is selected from the group consisting of Li⁺,Na⁺, K⁺, Rb⁺, Be²⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Sc³⁺, Y³⁺, Ti⁴⁺, Zr⁴⁺, Hf⁴⁺,V⁴⁺, V³⁺, V²⁺, Nb³⁺, Ta³⁺, Cr³⁺, Mo³⁺, VV³⁺, Mn³⁺, Mn²⁺, Re³⁺, Re²⁺,Fe³⁺, Fe²⁺, Ru³⁺, Ru²⁺, Os³⁺, Os²⁺, Co³⁺, Co²⁺, Rh²⁺, Rh⁺, Ir²⁺, Ir⁺,Ni²⁺, Ni⁺, Pd²⁺, Pd⁺, Pt²⁺, Pt⁺, Cu²⁺, Cu⁺, Ag⁺, Au⁺, Zn²⁺, Cd²⁺, Hg²⁺,B³⁺, B⁵⁺, Al³⁺, Ga³⁺, In³⁺, Tl³⁺, Si⁴⁺, Si²⁺, Ge⁴⁺, Ge²⁺, Sn⁴⁺, Sn²⁺,Pb⁴⁺, Pb²⁺, As⁵⁺, As³⁺, As⁺, Sb⁵⁺, Sb³⁺, Sb⁺, Bi⁵⁺, Bi³⁺, Bi⁺, andcombinations thereof.

Typically, the cluster has formula M_(m)X_(n) where M is the metal ion,X is selected from the group consisting of Group 14 through Group 17anion, m is an number from 1 to 10, and n is a number selected to chargebalance the cluster so that the cluster has a predetermined electriccharge

Preferably, X is selected from the group consisting of O²⁻, N³⁻ and S²⁻.Preferably. M is selected from the group consisting of Li⁺, K⁺, Na⁺,Cs⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, V²⁺, V³⁺, V⁴⁺, V⁵⁺, Mn²⁺, Re²⁺, Fe²⁺, Fe³⁺,Ru³⁺, Ru²⁺, Os²⁺, Co²⁺, Rh²⁺, Ir²⁺, Ni²⁺, Pd²⁺, Pt²⁺, Cu²⁺, Zn²⁺, Cd²⁺,Hg²⁺, Si²⁺, Ge²⁺, Sn²⁺, and Pb²⁺. More preferably M is Zn²⁺ and X isO²⁻.

Typically, the multidentate linking ligand has 6 or more atoms that areincorporated in aromatic rings or non-aromatic rings. Preferably, themultidentate linking ligand has 12 or more atoms that are incorporatedin aromatic rings or non-aromatic rings. More preferably, the one ormore multidentate linking ligands comprise a ligand selected from thegroup consisting of ligands having formulae 1 through 27:

wherein X is hydrogen, —NHR, —N(R)₂, halides, C₁₋₁₀ alkyl, C₆₋₁₈ aryl,or C₆₋₁₈ aralkyl, —NH₂, alkenyl, alkynyl, —Oalkyl, —NH(aryl),cycloalkyl, cycloalkenyl, cycloalkynyl, —(CO)R, —(SO₂)R, —(CO₂)R—SH,—S(alkyl), —SO₃H, —SO³⁻M⁺, —COOH, —COO⁻M⁺, —PO₃H₂—, —PO₃H⁻M⁺, —PO₃²⁻M²⁺, or —PO₃ ²⁻M²⁺, —NO₂, —CO₂H, silyl derivatives; boranederivatives; and ferrocenes and other metallocenes; M is a metal atom,and R is C₁₋₁₀ alkyl.

In one embodiment, the multidentate linking ligand comprises a ligandhaving formula 3 previously described. In another embodiment, themultidentate linking ligand comprises a ligand having formula 18(“BTB”). In a further embodiment, the multidentate linking ligandcomprises a ligand having formula 14.

A second aspect of the present invention provides an apparatus forseparating a metal organic framework (MOF) from a MOF containingsolution, comprising:

a housing having a reservoir capable of receiving a MOF containingsolution;

a high frequency ultrasound transducer operatively connected to thereservoir and capable of applying frequencies of at least 20 kHz,preferably between 20 kHz to 4 MHz, more preferably 500 kHz to 2 MHz,more preferably between 800 kHz and 2 MHz, and yet more preferablybetween 1 MHz and 2 MHz to the MOF containing solution; and

an acoustic reflector surface spaced apart from the transducer withinthe housing, the transducer, in use, being operated to reflect saidapplied high frequency ultrasound off the acoustic reflector surface,said acoustic reflector surface being spaced away from the highfrequency ultrasound transducer such that a standing wave is formedthrough constructive interface.

The apparatus of this second aspect of the present invention comprises aseparation apparatus for metal-organic frameworks. In use, a highfrequency ultrasound is applied to the MOF containing solution to effectseparation of the MOF from the solution. The apparatus can also be usedfor a washing or purification method. The MOF includes at least onecontaminant and the apparatus is used to separate those one or morecontaminants from the MOF.

Once again, the frequency of the applied high frequency ultrasound isimportant in the function and effect of the separation. Whilst thepreferred frequency depends on factors such as MOF composition, particlesize, solution composition and the like, the general ranges of appliedhigh frequency ultrasound the transducer is capable of applying to a MOFcontaining solution are as follows: In some embodiments, the appliedhigh frequency ultrasound is between 20 kHz to 4 MHz, preferably 500 kHzto 2 MHz, more preferably between 800 kHz and 2 MHz, and yet morepreferably between 1 MHz and 2 MHz. In some embodiments, the appliedhigh frequency ultrasound is greater than 1 MHz, preferably between 1MHz and 10 MHz, and more preferably between 1 and 4 MHz.

The Applicant considers that the size, material and/or geometry of thereactor vessel may have an effect on the outcome (degree, efficiency orthe like) of ultrasonic and/or megasonic separation process of MOFs.Similarly, the positioning, arrangement and alignment of transducerswithin a separation apparatus may have an effect on the outcome (degree,efficiency or the like) of megasonic separation process of MOF.

The transducer can be positioned in any suitable location in relation tothe housing to apply the megasonic frequencies to the MOF containingliquid received within the reservoir. In some embodiments, the housingcomprises a container including at least one wall position to contactthe MOF containing solution, and the transducer is high frequencyultrasound transducer is position within the reservoir or in engagementwith the at least one wall. In each case, the transducer is operable toapply ultrasonic and/or megasonic frequencies to a MOF containingsolution housed in the reservoir.

The transducer can comprise any suitable high frequency ultrasoundtransducer. In some embodiments, the high frequency ultrasoundtransducer comprises a plate transducer.

The acoustic reflection of the applied frequencies assists the MOFseparation process. Accordingly, in some embodiments the housingincludes at least one reflector surface designed to reflect the appliedmegasonic frequencies within the reservoir. The transducer is operatedto apply a high frequency ultrasound to the MOF containing solution andto reflect said applied ultrasound from the acoustic reflector surface.The use of an acoustic reflector surface assists in the formation of astanding wave field required to form pressure nodes where particles arecollected for cleaning or separation. This substantially separates theMOF from solution as an aggregated sediment which settles out ofsolution.

The acoustic reflector surface is generally located in front of thetransducer, and spaced apart from that transducer. In some embodiments,the transducer is located proximate or at one wall or side of thehousing, and the acoustic reflector surface is located proximate or atan opposite wall or side of the housing.

It should be appreciated that the apparatus of this second aspect caninclude all of the features discussed above in relation to the firstaspect of the present invention. Similarly, the method of the firstaspect of the present invention can utilise an apparatus of the secondaspect of the present invention therein.

A third aspect of the present invention provides a process of producinga metal organic framework (MOF), comprising:

forming a MOF in a reactor; and

isolating, washing and/or purifying the MOF using an apparatus accordingto the second aspect of the present invention.

A fourth aspect of the present invention provides a system for producinga metal organic framework (MOF), comprising:

a reactor for forming a MOF from precursor materials; and

an apparatus for washing and/or purifying the MOF according to thesecond aspect of the present invention.

Any suitable reactor can be used in the third and fourth aspects of thepresent invention. In some embodiments, the reactor is a batch reactor,for example a stirred reactor. In other embodiments, the reactor is acontinuous flow reactor. In preferred embodiments, the reactor comprisesa coiled continuous flow reactor. It should be appreciated that the MOFcan be treated in the apparatus according to the second aspect of thepresent invention and can either be provided in solution, for example amother liquid produced from the reactor, or be introduced/added into asolvent or liquid dispersant such as water, ethanol, methanol, DMF orthe like to form a MOF containing solution suitable for use in saidapparatus.

A fifth aspect of the present invention provides a method of separatinga metal organic framework (MOF) from at least one contaminant,comprising:

providing a MOF containing solution which includes a MOF and at leastone contaminant;

contacting the MOF containing solution with an acoustic reflectorsurface such that, any high frequency ultrasound applied within the MOFcontaining solution reflects off the acoustic reflector surface suchthat a standing wave is formed through constructive interference; and

applying a high frequency ultrasound of at least 20 kHz, at least 400kHz, preferably between 20 kHz to 4 MHz, more preferably 500 kHz to 2MHz, yet more preferably between 800 kHz and 2 MHz, and yet morepreferably between 1 MHz and 2 MHz to the MOF containing solution,

thereby substantially separating the contaminant from the MOF.

A sixth aspect of the present invention provides a method of activationof a metal organic framework (MOF), comprising:

providing a MOF containing solution;

contacting the MOF containing solution with an acoustic reflectorsurface such that, any high frequency ultrasound applied within the MOFcontaining solution reflects off the acoustic reflector surface suchthat a standing wave is formed through constructive interference; and

applying a high frequency ultrasound of at least 20 kHz, preferably atleast 400 kHz, preferably between 20 kHz to 4 MHz, more preferably 500kHz to 2 MHz, yet more preferably between 800 kHz and 2 MHz, and yetmore preferably between 1 MHz and 2 MHz to the MOF containing solution,

thereby improving the surface area and activation properties of the MOF.

A seventh aspect of the present invention provides a method of improvingthe surface area of a metal organic framework (MOF), comprising:

providing a MOF containing solution;

contacting the MOF containing solution with an acoustic reflectorsurface such that, any high frequency ultrasound applied within the MOFcontaining solution reflects off the acoustic reflector surface; and

applying a high frequency ultrasound of at least 20 kHz, preferably atleast 400 kHz, preferably between 20 kHz to 4 MHz, more preferably 500kHz to 2 MHz, yet more preferably between 800 kHz and 2 MHz, and yetmore preferably between 1 MHz and 2 MHz to the MOF containing solution,

thereby improving the surface area of the MOF.

Again, for each of the fifth, sixth and seventh aspects of the presentinvention, an acoustic reflector surface can be used to assist theformation of pressure nodes with the MOF containing solutions. In suchembodiments, the method further includes the step of:

contacting the MOF containing solution with an acoustic reflectorsurface such that, any high frequency ultrasound applied within the MOFcontaining solution reflects off the acoustic reflector surface.

Again, this step of contacting the MOF containing solution with anacoustic reflector surface can comprise any arrangement in which the MOFcontaining solution is contacted or otherwise has a fluid connectionwith the reflector surface. In some embodiments, the step of contactingthe MOF containing solution with an acoustic reflector surface comprisespositioning or otherwise providing the acoustic reflector surface withinthe MOF containing solution. The acoustic reflector surface ispreferably spaced away from the source of the high frequency ultrasoundapplied within the MOF containing solution that it reflects off theacoustic reflector surface such that a standing wave is formed throughconstructive interference. In this respect, the reflected sound wavesare able to interact with the original transmitted wave. If thereflected and the transmitted wave are in phase, i.e. the peaks andtroughs of the waves are aligned, then constructive interference willoccur leading to resonance. With this occurrence, pressure nodes andanti-nodes will form along the path of the interacting sound waves atdistances equal to multiples of half the wavelength of the waves.

In these and other embodiments of the present invention, the MOFcontaining solution is preferably provided into a housing containing ahigh frequency transducer and an acoustic reflector surface, thetransducer and the acoustic reflector surface being spaced apart withinthe housing; and the transducer is operated to apply a high frequencyultrasound of at least 20 kHz, preferably between 20 kHz to 4 MHz,preferably 500 kHz to 2 MHz, more preferably between 800 kHz and 2 MHz,and yet more preferably between 1 MHz and 2 MHz to the MOF containingsolution thereby substantially separating the MOF from solution as anaggregated sediment which settles out of solution. The use of anacoustic reflector surface assists in the formation of a standing wavefield required to form pressure nodes where particles are collected forcleaning or separation.

The BET surface area of the MOF is preferably improved at least 20%, andmore preferably 30% compared to a centrifuge washed MOF as it removesunreacted reagents trapped within the pores of the MOF.

Activation of a MOF using this aspect of the present invention is animportant step for cost-effective and green production of MOFs assimilar surface areas have only been obtained using laboratory scalemethods that would be expensive at large scale, namely by usingsupercritical ethanol or calcination up to 330° C.

It should be appreciated that the apparatus of this fifth, sixth andseventh aspect can include all of the features discussed above inrelation to the first and second aspects of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described with reference to thefigures of the accompanying drawings, which illustrate particularpreferred embodiments of the present invention, wherein:

FIG. 1A illustrates the nodal planes of acoustic resonant wave of (a)node and (b) antinode sections of a wave. Particle agglomeration occursat either the nodes or anti-nodes of the wave.

FIG. 1B illustrates the direction of secondary acoustic radiation forceon two particles.

FIG. 1C illustrates various alignments of transducers within a housingfor use in the method and apparatus of the present invention, namely a)Single transducer with vertical alignment, b) single transducer withhorizontal alignment, c) dual transducers with vertical and parallelalignment, d) dual transducers with vertical and perpendicularalignment.

FIG. 1D illustrates (a) vertical and (b) horizontal alignment of thetransducer and/or reflector for megasonic separation.

FIG. 2A provides a schematic representation showing the general flowreactor setup for the production of metal-organic framework solutionswhich are subsequently treated using the treatment apparatus accordingto the present invention.

FIG. 2B provides a schematic representation showing the different stagesof the continuous flow process for MOFs production: synthesis, washing,separation and drying.

FIG. 2C provides a) a photograph of a first separator set-up with a highfrequency system according to an embodiment of the present invention; b)a photograph of one 2000 kHz plate transducer used in the reactor set upshown in (a); and c) a schematic of a standing wave pattern formed bythe superimposition of a reflected sound wave within the megasonicseparator shown in (a).

FIG. 3A provides three photographs of solution being treated in theseparator shown in FIG. 2C at specific times (1 minute, 4 minutes and 10minutes) during a process according to one embodiment of the presentinvention.

FIG. 3B provides a comparison of SEM images of (a) and (b) Al-fum; (d)and (e) MIL-53 using conventional centrifuge separation on the left andmegasonics on the right; (f) and (g) Comparison of the XRD patterndiffraction of the megasonic and centrifuge product compared to acalculated pattern for Al-fum and MIL-53 respectively.

FIG. 3C provides a thermogravimetric analysis of Al-Fumarate and MIL-53(Al) (heating rate: 5° C./min).

FIG. 4A provides a photographic comparison and a comparison plot of thebackscattering and transmission data of the supernatant collected fromthe first separation of the MOF containing solution using centrifuge andapplied frequencies (megasonic) for (A) Al-Fumarate supernatant; and (B)MIL-53 (Al) supernatant.

FIG. 4B provides representation of BET surface area, SA_(BET) showingthe difference between the product isolated with megasonic andcentrifuge for (a) Al-fum; and (b) MIL-53, respectively.

FIGS. 5 and 6 shown the experimental set up of a second separator set-upwith a high frequency system according to an embodiment of the presentinvention.

FIG. 7 provides a plot of obtained supernatant volumes followingmegasonic separation of Al-Fum MOF for various frequencies. The totalvolume of MOF containing solution is 500 mL

FIG. 8 provides a plot of the mass faction vs reflector distance orspacing from the transducer.

FIG. 9 provides a plot of volume fraction vs reflector distance orspacing from the transducer.

FIG. 10 provides a) Schematic of inlet and outlet flows into prototypeMOF separation and activation vessel. b) The ultrasonic continuousreactor set-up with a high frequency system using one 1 MHz platetransducer.

DETAILED DESCRIPTION

The present invention provides a new separation apparatus, system andmethod that can separate a metal-organic framework (MOF) from asolution. The method has also been found to purify the MOF, removingcontaminants from the pores of the MOF and also improve the surface areaof the treated MOF, producing a purified MOF having a higher surfacearea than comparable commercially available samples.

In this regard, the Inventors have surprisingly found that the use ofultrasonic and/or megasonic frequencies not only separates MOFmaterial/particles from other components in a solution, but alsopurifies and activates the separated MOF. A MOF is extremely porous andtherefore contaminant species in a solution can be trapped or otherwiselocated in these pores. The method, system and apparatus of the presentinvention have been found to substantially remove any contaminants fromthe pores of MOF material treated with the megasonic method andapparatus. This produces a desirable substantially pure MOFs which ishighly scalable.

Furthermore, the Inventors have also surprisingly found that the use ofultrasonic, preferably megasonic frequencies also improves the surfacearea of the final product, acting as an alternate process to the timeconsuming and costly calcinations traditionally used for surface areaimprovement. The process can therefore assist in maintaining MOF productquality i.e. porosity, thermal and chemical stability.

The present invention can therefore permit large-scale production ofMOFs at drastically reduced costs, allowing commercialisation of theseMOFs for many potential real world applications. The present inventionprovides a fast, cost-effective, lowered environmental impact strategyto produce high-quality MOFs at a large scale.

The present invention uses megasonics technology for separation andactivation of metal organic frameworks. The Inventors believe that thisis the first time that ultrasonic frequencies and more particularlymegasonic frequencies have been used for the separation of MOFs.Furthermore, the Inventors believe that this is the first time thatmegasonics has been used for the purification/activation of porousmaterials, and in particular MOFs.

Separation according to the present invention applies >20 kHz, in somecases >400 kHz, preferably between 20 kHz to 4 MHz, preferably 500 kHzto 2 MHz, more preferably between 800 kHz and 2 MHz, and yet morepreferably between 1 MHz and 2 MHz high frequency ultrasound to create astanding wave, i.e. regions of minimal pressure (nodes) and maximalpressure (antinodes) within a liquid filled separation chamber. Whilstnot wishing to be limited to any one theory, the Inventor's considerthat when using this method, suspended particles or droplets migratespecifically towards one of these two regions due to acoustic radiationforces, based on their density and compressibility. In general, theaggregated MOFs are slightly denser than the surrounding fluid, andmigrate towards the pressure nodes. This gathering of MOF materialenhances the tendency to form larger aggregates which then sediment at agreatly accelerated rate to the bottom of the separation chamber, wherethey can be collected.

The process and apparatus of the present invention also has the abilityto achieve specificity of separation based on particle size by tuning ofthe operation parameters such as frequency and energy density.

Furthermore, the application of the sound waves dramatically enhancesthe rate of separation, and hence reduces the chemical requirement andenvironmental footprint of conventional separation processes such asflocculation and sedimentation. This can be seen in a comparison ofcleaning and separation techniques shown in Table 1:

TABLE 1 Comparison of cleaning and separation techniques using MIL-53 asa control MOF Activation Temperature SA_(BET) Methods Separation time (°C.) (m²/g) Centrifuge¹ Yes 1 min Room Temp 806 Megasonics¹ Yes 10 minRoom Temp 1183 Ultrasound in the No 40 min 70 787 presence of Amides ²Ultrasound in the No 1 hour 70 1425 presence of Amides ² Supercritics³Yes 4 hours 250° C. and 100 bar 1010 Calcination ² No 72 hours  325 1256¹Applicant's experiments. ² M. Gaab, N. Trukhan, S. Maurer, R. Gummarajuand U. Müller, Microporous Mesoporous Mater., 2012, 157, 131-136. ³P. A.Bayliss, I. A. Ibarra, E. Pérez, S. Yang, C. C. Tang, M. Poliakoff andM. Schröder, Green Chem., 2014, 16, 3796.

Ultrasonics and/or megasonic operation involves no moving parts, and canhave a low surface area of contact with the fluid providing a lowercapacity for fouling, and ease of cleaning. A separator according to thepresent invention essentially comprises a housing or container in whicha liquid reservoir can be formed. The liquid reservoir is filled with aMOF containing solution. A high frequency transducer, such as a platetransducer is either submerged in the liquid filled reservoir or engagedwith a wall of reservoir to project megasonic frequencies through theMOF containing solution for a certain length of time to effects thedesired separation of MOF from solution and/or separation ofcontaminants from the MOF into the solution.

The Applicant considers that the size, material and/or geometry of thereactor vessel may have an effect on the outcome (degree, efficiency orthe like) of the separation process of MOFs using the present invention.Similarly, the positioning, arrangement and alignment of transducerswithin a separation apparatus may have an effect on the outcome (degree,efficiency or the like) of the separation process of MOFs.

The Applicant notes that ultrasonics and megasonics are a well knowseparation technique for particles, particularly in the biotechnologyand food processing areas. Previous applications of ultrasonics andmegasonic involved liquid/liquid and solid/liquid separation especiallyin food processing (milk fat separation and palm oil separation).However, the Inventors are not aware of any previous published workusing ultrasound, in particular megasonics, for the combined separation,washing, and/or activation of any porous material.

The inventors believe that the ultrasonic and megasonic ranges of thepresent invention provide at least one of surface area improvement,separation and/or washing properties for MOF containing solutions. Thedifference between ultrasonic and megasonics lies in the frequency thatis used to generate the acoustic waves. Ultrasonic uses lowerfrequencies (20 kHz to 400 kHz) and produces random cavitations.Megasonic uses higher frequencies frequency (>0.4 MHz to several MHz)and produces controlled and smaller cavitations which allows theseparation of nanocrystals (in our case, the MOFs). Furthermore, highermegasonic frequencies do not cause the violent cavitation effects foundwith ultrasonic frequencies. This significantly reduces or eliminatescavitation erosion and the likelihood of surface damage to the productbeing cleaned.

Again, it should be appreciated that the MOF containing solutiontypically comprises a mixture or suspension of the MOF particles withinthe solution. In this regard, the MOF comprises a solid component orparticles which are dispersed throughout the liquid of that solution.For ease of reference, this suspension of MOF particles in solution willbe referred to as a MOF containing solution in this specification.

The separation method and apparatus of the present invention utilises areflector within the separation arrangement. Without wishing to belimited to any one theory, the Inventors have found that introducing areflector allows a standing wave to be formed through constructiveinterference. As the soundwaves from the transducer reach the reflector,they are reflected where they may interact with the original transmittedwave. If the reflected and the transmitted wave are in phase, i.e. thepeaks and troughs of the waves are aligned, then constructiveinterference will occur leading to resonance. With this occurrence,pressure nodes and anti-nodes will form along the path of theinteracting sound waves at distances equal to multiples of half thewavelength of the waves.

Particles in the vicinity of these nodes and anti-nodes experience aseries of forces as a result of the sounds resonance, these are known asthe primary acoustic radiation force and the Bejerknes force (orsecondary acoustic radiation force). These forces will cause the solidMOF species to agglomerate at the nodes or anti-nodes, increasing therate of settling (given their density is greater than the fluid). Aftertime, the solids will deposit within a bed forming solids leansupernatant.

Acoustic cavitation within a liquid is a possibility when operating withmegasonics; the intense pressure of the ultrasonic waves has thecapability to cause dissolved gases to exit solution leading to theformation of bubbles. Upon collapsing, extreme temperatures andpressures can be achieved of up to 10,000 K and many hundreds of barrespectively.

Increasing the ultrasounds frequency has the effect of smaller bubblesas they collapse faster under the more intense conditions. These smallerbubbles collapse with less energy, leading to less cavitation. The rangeof ultrasonic frequencies over which cavitation can be observed istypically between 20 kHz to 4 MHz.

The primary acoustic radiation force (F_(ac)) is a second order,non-linear force that acts upon particles within an acoustic standingwave field. Momentum is transferred from the soundwave to the solid,allowing its manipulation in the direction parallel to the soundwavespropagation. For an ideal standing wave, the time averaged force in thedirection of the sounds propagation can be described by:

$\begin{matrix}{F_{ac} = {{- \frac{4\pi}{3}}R^{3}{kE}_{ac}\phi\;{\sin\left( {2{kx}} \right)}}} & \lbrack 2.1\rbrack \\{k = \frac{2\pi}{\lambda}} & \lbrack 2.2\rbrack \\{\phi = {\frac{{5\rho_{p}} - {2\rho_{m}}}{{2\rho_{p}} + \rho_{m}} - \frac{\beta_{p}}{\beta_{m}}}} & \lbrack 2.3\rbrack\end{matrix}$

Where:

-   -   R Is the Particle Radius    -   k is the wavenumber    -   E_(ac) is the specific energy density    -   ϕ is the acoustic contrast factor    -   ρ_(m), ρ_(p) is the density of the medium or particle        respectively    -   β_(m), β_(p) is the compressibility of the medium or particle        respectively

The compressibility of the particle can be estimated from the followingrelation:

$\begin{matrix}{\beta_{p} = \frac{1}{\rho_{p}c^{2}}} & \lbrack 2.4\rbrack\end{matrix}$

The acoustic contrast factor determines whether the solids will bedriven to the nodal or anti-nodal planes of the resonance field. Anegative factor (<0) indicates the particles will be displaced to thepressure anti-nodes within the field, whilst a positive factor (>0)indicates the particles will be displaced to the pressure node planes(see FIG. 1A). An acoustic contrast factor of 0 means separation cannotbe obtained within the fluid medium via primary radiation forces.Typically particles have densities higher, and compressibility's lowerthan the surrounding fluid and will move to the pressure anti-node ofthe system.

The secondary acoustic radiation force (F_(sec)), also known as theBejerknes force acts upon the particles within the nodal plane pushingthem towards each other and resulting in aggregation. Outside of thenodal planes, at angles other than 90°, the particles are repulsed asshown in FIG. 1B. The force can be described by the following:

$\begin{matrix}{F_{\sec} = {4\pi\; R_{p_{1}}^{3}{R_{p_{2}}^{3}\left( {\frac{\left( {{3\mspace{14mu}\cos^{2}\mspace{14mu}\theta_{r}} - 1} \right)\left( {\rho_{p} - \rho_{m}} \right)^{2}v^{2}}{6\rho_{m}d^{4}} - \frac{\left( {\beta_{p} - \beta_{m}} \right)^{2}\rho_{m}\omega^{2}p^{2}}{9d^{2}}} \right)}}} & \lbrack 2.5\rbrack\end{matrix}$

-   -   Where:    -   R_(p) ₁ , R_(p) ₂ is the radius of the two interacting particles    -   θ_(r) is the angle of the connection between the two particles,        relative to the direction of sound propagation    -   p is the acoustic pressure    -   v is the velocity of the wave in a 1-D acoustic plane    -   d is the centre to centre distance between the particles    -   ω is the angular frequency of the oscillation

The force results from the scattering of the sound waves fromneighbouring particles within a sound field.

Transducer alignment needs to be considered in the positioning of thetransducer 40 and reflector 42 within a housing 44 of a separationapparatus according to the present invention to generate standing waves.A vertical or horizontal alignment between the transducer 40 andreflector 42 refers to whether the transducer 40 or reflector 42 ispositioned in a way that the nodal/antinodal bands are aligned in thevertical or horizontal plane (see FIGS. 1C and 1D).

Where gravity is a necessary mechanism for enhanced separation (i.e.,product collects at the top or bottom of the container), a verticalalignment (see FIG. 1D(a)) is more amenable to separation since onceproduct is aggregated to a sufficient size, the product can sediment orrise rapidly due to buoyancy. Vertical alignment bands 46 are formedwhen applying ultrasound in pulses.

A horizontal alignment (see FIG. 1D(b)), however, hinders this naturalrise/fall since the product must pass through aligned bands that can“trap” the aggregated material prior to eventual rising/falling beyondthe active processing region. Applying ultrasound in pulses canintermittently release product trapped in the horizontally aligned bands47, but decreases the energy input to the system per unit time.

The Inventors have found that both types of alignments can besuccessfully applied for use of MOFs activation and separation. However,due to the above outlined reasons, the horizontal alignment requiresadditional time for settling the MOF crystals at the bottom of thevessel.

Sound wave attenuation may also be a factor to consider during theseparation process. As the wave propagates through the medium, itsenergy will dissipate into mainly the form of heat. The rate at whichthis attenuation occurs depends upon the medium (both the fluid and thesuspended solid) which the sound is travelling through in addition tofrequency. Higher frequencies experience more rapid attenuation in amedia; hence lower frequencies may be favoured with care taken to avoidcavitation.

In achieving separation, care must be taken to ensure the distance overwhich the sound must travel (to reflector and back) is not such thatexcessive attenuation occurs, resulting in weak acoustic separationforces. Whilst the volume which can be separated may increase with alarger separation distance between transducer and reflector, the rate ofseparation may be adversely affected in doing so. Sound wave attenuationis proportional to frequency, such that increasing the frequency willreduce the distance over which the separation will be effective.

Particle size may also have significant effect upon the separationprocess given both the primary and secondary acoustic radiation forcesare proportional to the cubed radius (Eqn. 2.1 and 2.5). Therefore alarger particle is more easily manipulated than a smaller particle;however particles that are too large could interrupt the standing fieldof the resonance wave.

Acoustic streaming occurs when the fluid bulk is set in motion due tothe sound wave oscillations which can overcome the manipulative acousticradiation forces acting on the nodal and anti-nodal separation planes.

Acoustic attenuation, amongst other mechanisms, tends to lead to theoccurrence of acoustic streaming. Attenuation scales with square of thefrequency of the sound wave, as described by:

$\begin{matrix}{\alpha = \frac{2{\mu\left( {2\pi\; f} \right)}^{2}}{3\rho_{m}c^{3}}} & \lbrack 2.6\rbrack\end{matrix}$

-   -   Where:    -   μ is the viscosity    -   ρ_(m) is the density of the medium

Hence increasing the frequency and amplitude of the sound wave willresult in more significant streaming which may severely hinder theseparation process.

EXAMPLES

The separation of two studied MOFs, aluminium fumarate (Al-fum) andaluminium terephthalate (MIL-53) using a Megasonic separation processand apparatus according the present invention, will now be exemplifiedby example. However, it should be appreciated that the present inventionis suitable for use with a large number of MOFs and should therefore notbe limited to the exemplified MOF structures in these examples. Theexamples provided can therefore be more generally applied to a widerange of MOFs.

Example 1—MOF Separation

MOF Synthesis

Aluminium fumarate (Al-fum) and aluminium terephthalate (MIL-53) weresynthesized using flow chemistry technology following the methodologytaught in Rubio-Martinez et al, (2014) “Versatile, High Quality andScalable Continuous Flow Production of Metal-Organic Frameworks”,Scientific Reports 4, Article number: 5443 doi:10.1038/srep05443(“Rubio-Martinez 2014”), the contents of which are to be understood tobe incorporated into this specification by this reference.

Aluminium fumarate (Al-fum) and aluminium terephthalate (MIL-53) wereused as each exhibit high thermal stability up to 450° C. and present areversible uptake/release of water provided by an octahedral aluminiumconfiguration and a strong Al—O bond. Both of these MOFs present verysimilar structures where the carboxylate groups of the correspondinglinkers lead to the formation of a 3D structure with rhombohedralchannels interconnected by infinite Al—OH—Al chains.

A schematic representation showing the general flow reactor setup forthe production of MOFs generally following Rubio-Martinez 2014 is shownin FIG. 2A. FIG. 2B shows another representative flow reactorconfiguration for MOF processing. Continuous flow scale-up synthesis wasperformed in a Salamander Flow Reactor (Cambridge Reactor Design Ltd.,Cottenham, UK). Briefly, it consists of a reactor 50 comprisingserpentine stainless steel tube (8 mm o.d., 6 mm i.d., 108 mL volume)and a thermostatically-controlled electrical heating system (ambient to150° C.). An inline back-pressure regulator, situated at the outlet ofthe reactor, allows fine tuning of the reactor pressure (up to 20 bar).Static mixer units are placed within the linear sections of the reactortubing to promote turbulent mixing and efficient heat transfer. TwinGilson 305 dual piston pumps 62, (flow rates between 0.5 mL/min and 50mL/min) provide the solvent and reagent feeds for the reactor system.Each separate precursor solutions of the organic ligand 55 and themetallic salt 57 are simultaneous pumped into a T-micro mixer 60 viapumps 62 using a commercially available flow chemistry synthesisplatform. The mixed solvent streams are combined and directed into thecoiled flow reactors 50. Each reactor coil 50 has its temperatureregulated to be constant and homogenous throughout the reaction,eliminating the possible temperature gradients often observed in batchreactors. Preferably, the solvent, preferably water and/or mixture ofwater and ethanol, is kept at a temperature from about 25° C. to about130° C. depending on the MOF synthesis. Typically, higher ligandconcentration provides increased yields, however, the risk of blockagein the flow reactor 50 is also increased.

In a typical reaction, two separate solutions of the precursors arepumped through a T-type static mixer to promote diffusion mixing of thereagent input streams. The combined reagent streams are then directedinto the heated reactor zone of the Salamander Flow Reactor for apredetermined residence time. On exiting the reactor, the MOF stream 65is cooled in an external heat sink unit, based on a coiled tube in awater bath (FIG. 2B). Then the stream passes through a back-pressureregulator, and is collected for the next process steps. If desired, thesolvent can be reused by recycling after the first separation stage.This is particularly attractive for recycling the unreacted ligand whichis usually the most expensive reactant, or when an expensive or toxicsolvent is used.

MOF stream 65 can be collected in a container or reservoir 70 which canthen be subsequently fed into a megasonic separator 100 according to thepresent invention. This device is illustrated in more detail in FIG. 2C.

Synthesis of Al-Fumarate

The general procedure described above was employed. An aqueous solutionof 0.35M Al₂(SO4)₃ 18H₂O and an aqueous solution of 0.7M of fumaric acidand 2M of NaOH solution were mixed under continuous flow conditions andheated in a tubular reactor. The synthesis was conducted at 65° C. usinga total flow rate of 90 mL·min⁻¹, giving a total residence time of 1.2min. The material was washed three times with fresh water and twice withethanol and dried in vacuum (500 mbar) for 8 hours at 40° C. Yield:100%.

S2.b. Synthesis of MIL-53 (Al)

The general procedure described above was employed. An aqueous solutionof 0.08M Al(NO₃)₃ and an aqueous solution of 0.08M of terephthalic acidand 0.24M of NaOH solution were mixed under continuous flow conditionsand heated in a tubular reactor. The synthesis was conducted at 140° C.using a total flow rate of 90 mL·min-1, giving a total residence time of1.2 min. The material was washed three times with fresh water and twicewith ethanol and dried in vacuum (500 mbar) for 8 hours at 40° C. Yield:83%.

MOF Separation Process

The MOF crystals were isolated from the solvent using a megasonicapparatus and process according to one embodiment of the presentinvention. A conventional centrifuge was used as a control reference.

The megasonic separator 100 is shown in FIG. 2C. The megasonic separator100 applies high frequency ultrasound to create a standing wave, i.e.regions of minimal pressure (nodes) and maximal pressure (antinodes)within a separation chamber 110 of megasonic separator 100.

FIG. 2C(a) shows the megasonic separator 100 set-up with a highfrequency system using one 2000 kHz plate transducer 105 (best shown inFIG. 2C(b)). All trials were conducted utilizing submersible stainlesssteel transducer plates (Sonosys Ultraschallsysteme GmbH, Neuenbuerg,Germany). The megasonic separator 100 essentially comprises a 1.1 Lstainless steel container. It should be noted that a clear polycarbonate6-litre container shown in the Figures was used initially to visualizethe separation process. However, normal operation and experiments wereperformed in a 1.1-litre stainless steel container (not pictured).

The illustrated clear polycarbonate 6-litre container is split into twosections, a 1.1 L treatment section 110 containing the transducer plate105 and an unprocessed section 112. The treatment section 110 andunprocessed section 112 are separated by a metallic (stainless steel)reflector plate 115 used to reflect the megasonic waves.

The plate transducer 105 was used for sonication at a frequency of 1 and2 MHz (290 W) in separate trials for 10 min. Each experiment consistedof filling the acoustic reactor with a diluted MOF solution (50% inwater) and immediately sonicating for 10 min. A control system, where noultrasound was applied, was simultaneously filled with a portion of thesame MOFs solution to observe the differences. In all experiments thetemperature increased up to 10° C., caused by acoustic energydissipation, therefore an ice bath is used during the experiments.

Before and after the application of ultrasound, 10 mL samples wereremoved to measure the ζ-potential of the MOFs. Using megasonics the MOFproduct was washed three times with fresh water and twice with EtOH.

FIG. 2C(c) shows the schematic of the standing wave pattern formed bythe superimposition of a reflected sound wave within the treatmentsection 110. The separation distance between adjacent nodes orantinodes, is half a wavelength. Depending on the specific density andcompressibility of the particles, they will collect either in the nodal(top, black dotted planes) as for the bright yellow particles orantinodal (bottom, red dotted planes) pressure planes as for the darkeryellow particles. As previously noted, suspended particles or dropletsmigrate specifically towards one of these two regions due to acousticradiation forces, based on their density and compressibility. Ingeneral, the aggregated MOFs are slightly denser than the surroundingfluid, and migrate towards the pressure nodes. As shown in FIG. 3, thisgathering of MOF material enhances the tendency to form largeraggregates which then sediment settles at a greatly accelerated rate tothe bottom of the separation chamber, where they can be collected.

FIG. 3A provides three photographs of a MOF containing solution beingtreated in a megasonic treatment apparatus 100 shown in FIG. 2C(a) atspecific times (1 minute, 4 minute and 10 minutes) during the megasonicseparation process described above. In the left or separationcompartment 110, the megasonic separation and purification process ofthe Al-MOF is shown. The right compartment 112 shows the same MOFcontaining solution without sonication. The settling of the MOF isclearly visible in the separation compartment 110 after 4 mins and 10mins compared to the cloudiness of the same MOF containing solutionwithout sonication shown in the right compartment 112.

The inventors believe that the precise mechanism may be related tochanges in the local density that the aggregates experience due tovarying size distribution of the nanosized MOF crystals. Generally, theinfluence of ultrasound on suspended particles depends on particle size,density and ultrasonic field. However, the resultant separation can befurther influenced by possible interactions between MOF particles whenthey collide (i.e. surface properties). The solvent properties will alsoinfluence the specific density of the particles in the field, so thatmay also affect the separation efficiency as well. For the MOFsstructures and solvent studied in these experiment, no appreciabledifferences were observed.

To determine the quality of the crystals, the MOFs separated withmegasonic treatment apparatus 100 and a standard lab-scale centrifugewere compared by XRPD and SEM measurements. X-Ray powder diffraction(XRPD) confirmed the crystallinity of Al-fum and MIL-53, showingidentical patterns to those of crystals synthesized by solvothermalmethods. Note that the Megasonics separation had no impact on thecrystallinity of the materials as demonstrated by identical patterndiffraction (See FIGS. 3B(f) and (g)). From the SEM images it wasobserved that the high-frequency treatment did not affect the size andshape distribution of the MOFs (See FIG. 3B). The thermo gravimetricanalysis (TGA) curves showed a continuous weight loss over thetemperatures ranges 50 to 100° C. due to water loss and thermalstability up to 450° C. (see FIG. 3C).

The scanning electron microscopy (SEM) images were collected on a Quanta400 FEG ESEM (FEI) at acceleration voltage of 0.2-30 kV. Infrared (IR)spectra were recorded on a Tensor 27FTIR spectrophotometer (Bruker). TheX-ray powder diffraction (XRPD) measurements were performed with anX'Pert Pro MPD diffractometer (Panalytical) over a 28 range of 5° to45°. The thermogravimetric analysis (TGA) was performed on aPerkin-Elmer STA-600 under a constant flow of N₂ at a temperatureincrease rate of 5° C./min. Zeta potential measurements were performedon a NanoZs Zetasizer from MALVERN whereas the Turbiscan measurementswere performed with the MA 2000 (Formulaction, Toulouse, France). Gasadsorption isotherms for pressures in the range 0-120 kPa were measuredby a volumetric approach using a Micrometrics ASAP 2420 instrument. Allthe samples were transferred to pre-dried and weighed analysis tubes andsealed with Transcal stoppers. Al-Fumarate and MIL-53 were evacuated andactivated under dynamic vacuum at 10⁻⁶ Torr at 140° C. for 8 hours.Ultra-high purity N₂, CH₄, H₂ and CO₂ gases were used for theexperiments. N₂ and H₂ adsorption and desorption measurements wereconducted at 77K. Surface area measurements were performed on N₂isotherms at 77K using the Brunauer-Emmer-Teller (BET) model withadsorption values increasing range of 0.005 to 0.2 relative pressureswhile the CH₄ adsorption and CO₂ adsorption measurements were done at273 and 298 K, respectively.

Example 2—Investigation into Changes in MOF Composition

In order to investigate whether megasonics separation introduces changesin the MOF composition, ζ-potential measurements were recorded aftereach washing step of Example 1 as shown Table 2.

TABLE 2 ζ- Potential of the Al-Fumarate and MIL-53 MOF material aftereach wash step using Megasonics using water as a dispersant. MOF washingprocess (Megasonics) ζ- potential (mV) Al-Fumarate flow reactor  +8.3 ±0.4 Al-Fumarate wash 1 in H₂O  +8.8 ± 0.0 Al-Fumarate wash 2 in H₂O +8.8 ± 0.1 Al-Fumarate wash 3 in H₂O  +8.9 ± 0.2 Al-Fumarate wash 4 inEtOH +10.6 ± 0.2 Al-Fumarate wash 5 in EtOH +11.3 ± 0.8 MIL-53 flowreactor +13.3 ± 0.4 MIL-53 wash 1 in H₂O +15.1 ± 0.5 MIL-53 wash 2 inH₂O +14.7 ± 0.3 MIL-53 wash 3 in H₂O +12.6 ± 0.5 MIL-53 wash 4 in EtOH+12.7 ± 0.2 MIL-53 wash 5 in EtOH +14.6 ± 0.1

No significant changes to the surface charge were observed, pointing toa separation that is based on reversible aggregation.

To determine the quality of the crystals, XRPD and SEM measurements ofthe MOFs separated with megasonics and by the standard lab-scalecentrifuge were compared. X-Ray powder diffraction (XRPD) confirmed thecrystallinity of our Al-fum and MIL-53, showing identical patterns tothose of crystals synthesized by solvothermal methods. It was observedby scanning electron microscope that the high-frequency treatment alsodoes not affect the size and shape distribution of the MOFs.

A comparison of the backscattering and transmission data of thesupernatant collected from the first separation of the MOF containingsolution using centrifuge and megasonics was undertaken as shown in FIG.4. As shown in FIG. 4, the recoverable MOF yield obtained with megasonicseparation compared to the conventional centrifuge method is 3% less foreach washing step. This difference can be attributed to the fact thatcentrifuge separation generates a higher G-force compared to thesettling by gravity in megasonics, which leads to a more effectiveremoval of the MOF material.

The measurements of the BET surface areas revealed that the MOFsseparated and washed with megasonics showed a drastic increase of 21%for the Al-Fum and 47% for MIL-53 over standard centrifuge washed MOF,which had BET values similar to literature (see Table 3 and FIG. 4B).

TABLE 3 Comparisons of the reaction time between MOFs synthesized byconvectional batch (using water as a reaction solvent) and by flowchemistry. BET surface areas, grams of MOF produced per 1 hour usingflow chemistry and STY. Full adsorption isotherms are provided in thesupplement information. STY Reaction Yield (Kg · m⁻³ · SA_(BET) time gh⁻¹ (%) d⁻¹) (m² g⁻¹) From reactor Al-fum 1.2 min 338.04 109.0 25,040 —MIL-53 1.2 min 50.68 112.8 3,754 — Centrifuge x 5 Al-fum 1.2 min 281.8890.9 20,880 890 MIL-53 1.2 min 42.14 93.8 3,121 806 Megasonics x 5Al-fum 1.2 min 225.07 72.6 16,672 1075 MIL-53 1.2 min 35.10 78.1 2,6001183 Commercial^(a) Al-fum 10.2 min 174 86 5339 1140 Literature^(b)MIL-53 4 hours 125 86 1300 1010 ^(a)M. Gaab, N. Trukhan, S. Maurer, R.Gummaraju and U. Müller, Microporous Mesoporous Mater., 2012, 157,131-136. ^(b)P. A. Bayliss, I. A. Ibarra, E. Pérez, S. Yang, C. C. Tang,M. Poliakoff and M. Schröder, Green Chem., 2014, 16, 3796.

The Inventors attribute this improvement to the enhanced mass transferthat arises from acoustic streaming during megasonic application thatpromotes the removal of the excess organic ligands molecules inside ofthe pores. This is an important step forward for cost-effective andgreen production of MOFs as similar surface areas have only beenobtained using laboratory scale methods that would be expensive at largescale, namely by using supercritical ethanol or calcination up to 330°C.

Example 3—MOF Characterisation and Separation Parameters

Megasonics Characterisation

The separation of the MOFs was carried out in the experimental set up200 shown in FIGS. 5 and 6. This separation arrangement comprised anopen-ended cylindrical glass vessel 202 with a capacity of 1 L mountedon a stand or base 203. A glass valve (sampling valve 204) was attachedto the lower vessel opening to allow easy sampling of the solids bed,post separation. A piezoelectric transducer 205 (E 805/T/M MeindhardtUltrasound Transducer) was positioned at the base of the cylinder, whichmade a water tight seal with the vessel 202. The transducer 205 wascapable of operating are three separate frequencies (578, 860, 1138 kHz)in either pulse or continuous configurations. However only continuouswas used for the experiments. Ultrasonic signals were generated using anultrasonic multi-frequency generator (Meindhardt, not illustrated) andpower output could be adjusted to a nominal value between 0 and 100%.The vessel 202 was filled with various volumes of MOF (208—FIG. 6) and astainless steel reflector 210 (thickness 0.5 cm) was positioned to be incontact with the slurry. The reflector 210 was connected to a steelthreaded rod 212 which could move freely through a lid 214, allowing theheight of the reflector 210 to be altered. The system also included acooling jacket 216 which where necessary used a flow of cooling water218 to cool the mixture contained in the vessel 202.

The power draw of the system 200 was determined using a standard powermeter (not illustrated), although obtaining an accurate reading wasdifficult given the large fluctuations at each frequency. The averagepower draw of the system 200 at each frequency was determined over a 10minute period of operation with a measurement taken at each minuteinterval from the power meter. At the completion of the 10 minute periodthe average power draw of the system at each frequency was determined.At frequencies of 1138, 860 and 578 kHz the power draw of the system wasfound to be 399±7, 422±2 and 408±17 W, with the baseline power draw(whilst no ultrasonics were generated) of 84 W. This suggests thetransducer's 210 power output for each of the frequencies (1138, 860 and578 kHz) were 315±7, 338±2 and 324±17 W.

Separation Optimisation

Several parameters were of interest when attempting to optimise theseparation of the MOF; these included frequency, and reflector height.

In order to determine the best frequency for MOF separation themegasonics vessel was first filled with 500 mL of clean (pre washed)Al-Fum and a frequency (1138, 860 or 578 kHz) was selected. Megasonicswere then applied for a period of 10 minutes at 100% power and areflector height of 14.5 cm, for the chosen frequency. At the end of thetreatment period, the megasonics were switched off and the slurryallowed to freely settle for a period of 15 minutes. Followingseparation, the supernatant was decanted using a peristaltic pump andthe volume was recorded before two solids samples were taken using thesolids sampling valve 204 (FIG. 5). The mass of each of the solidssamples was weighed before being dried overnight. The solids fraction(mass of solids per mass of slurry) was determined for each sample inorder to characterise the degree of separation achieved. Finally, theprocess was repeated for the remaining two frequencies. The experimentwas then repeated for each frequency without the use of a reflector.

Again the same Al-Fum MOF was left overnight before tests wereundertaken to investigate the effect of reflector height. The vessel wasfilled to heights of 11.5, 14.5, 20.5 and 26.5 cm in succession, withmegasonic treatment of 10 minutes at 100% and 1138 kHz. The MOF was onceagain allowed to settle for a period of 15 minutes before supernatantvolume and bed mass fractions were taken in the same way as earlierdescribed.

Activation of MOF

Following separation optimisation, transition was made from cleanpre-washed MOF to reactor slurry in order to characterise the effects ofthe megasonics on both the activation process as well as any changes tothe MOFs structure. In order to achieve this, 500 mL of Al-Fum reactorslurry was diluted to a 3 L batch. From this batch 500 mL was added tothe megasonics vessel 202 and sonicated for 10 minutes at 100% poweroutput with a frequency of 1138 kHz. At the conclusion of the treatment,the slurry was allowed to freely settle for 15 minutes before a solidssample was taken (8 mL) from the top layer (1 cm) of the bed. Aperistaltic pump was then used to remove a total of 100 mL ofsupernatant. 100 mL of fresh water (milliQ grade) was added to thevessel prior to removal of the slurry from the megasonics vessel 202.The slurry was stirred briefly and a small sample (<5 mL) was taken forSEM analysis before being re-added to the vessel 202 for a furthertreatment. This was repeated until a total of 4 cycles (washes) had beencompleted. At the conclusion of the final washing stage, additionalsolids were removed for BET analysis.

As a comparison, 500 mL of the same batch of diluted Al-Fum wascentrifuged at 4500 RPM for 5 minutes. A small solids sample was takenfor XRD analysis prior to the removal of 100 mL of supernatant. This wasreplaced with 100 mL of fresh water; the separated MOF was agitatedbefore the process was repeated for a total of 4 washes. At thecompletion of 4 washes, additional solids were removed for the purposeof BET analysis

Post reactor Mil-53 slurry was then used in place of Al-Fum and the sametests were repeated although in this case separation was achievedwithout initial the dilution (500 mL to 3 L) given its already dilutenature.

Results and Discussion

Optimal Frequency—

Carrying out treatments at each frequency, the greatest MOF separation(and hence greatest amount of separation) was achieved when treatmentoccurred with a frequency of 1138 kHz rather than 860 or 578 kHz (FIG.7). This is supported by the obtained mass fractions within the bedwhich show the highest concentration of MOF present within the bed when1138 kHz are used. On the other hand, the same experiments performedwithout the reflector did not achieve separation, which indicates thatin order to isolate the MOF the formation of standing waves is required.The standing wave generates regions of minimal pressure (nodes) andmaximal pressure (antinodes) within a separation chamber. Due to localacoustic radiation forces, suspended MOF particles migrated specificallytowards the regions of minimal pressure (nodes) and maximal pressure(antinodes). This gathering of MOF material enhanced the tendency toform larger aggregates, which then settle at a greatly accelerated rateto the bottom of the separation chamber, where they were collected.

Effect of Reflector Height—

Various reflector heights (11.5, 14.5, 20.5 and 26.5 cm) were analysedfor their effect on the degree of separation. Tests were carried out inthis case at a frequency of 1138 kHz given that is was expected to bethe most highly attenuating (lowest operable distance, Section 2.1.3)and was observed to give the highest degree of separation. From FIGS. 8and 9, it appears that the greatest degree of separation occurs at amid-range reflector height of approximately 14.5 cm. At lower heights(11.5 cm) lower bed mass fractions were observed as well as lower volumefractions (volume of supernatant per total volume). This could bepotentially due to increased streaming given the lower volume of slurrypresent which would in turn reduce the degree of separation (as outlinedabove).

At greater reflector heights (20.5 cm and 26.5 cm), it was found thatonce again separation was reduced when compared to separation when usinga reflector at 14.5 cm. This is undoubtedly due to acoustic attenuationwhich sees the amplitude of the sound die out at longer treatmentdistances. Although 14.5 cm was identified as the optimal reflectorheight, it did not lead to significant increases in the degree ofseparation and successful separations were carried out all heights (11.5cm-26.5 cm).

Activation of Metal-Organic Frameworks—

Comparing the effects of megasonic activation to centrifugation (Table4), it is clear that significantly higher surface area has been achievedwhen megasonic treatment is applied. The measurements of the BET surfaceareas revealed that the MOFs separated and activated with megasonicsshowed an increase of 25% after 10 minutes (for the Al-Fum over standardcentrifuge washed MOF, which had BET values similar to literature). Weattribute this improvement to the enhanced mass transfer that arose fromacoustic streaming during megasonic application promoting the removal ofexcess organic ligands molecules from the pores of the MOF crystals.

TABLE 4 Water washing comparison, BET isotherms for centrifuge andmegasonic Al-Fum washes at different times at 1138 kHz SA_(BET) Methods(m²/g) Centrifuge 880 Megasonics 1 min 907 Megasonics 5 min 916Megasonics 10 min 975 Megasonics 10 min in ETOH 1075

Example 4—Continuous Megasonics Operation

A prototype Continuous MOF separation process arrangement 300 accordingto the present invention was trialled as illustrated in FIG. 10. In thisregard, FIG. 10 provides a) schematic of inlet 305 and outlet flows 310into prototype MOF separation and activation vessel 301 which encloses aMOF processing region 307 and b) a photograph of prototype MOFseparation and activation vessel 301 including an ultrasonic continuousreactor set-up with a high frequency system using one 1 MHz platetransducer 312. As shown in both FIGS. 10(a) and 10(b), the processarrangement 300 comprises a solution inlet 305 which includes a fluidpump 316 to pump unprocessed MOF containing solution from feed flask 302into the prototype MOF separation and activation vessel 301 whichincludes a MOF processing region 307 containing a fluid reservoir, atransducer plate 312 for applying the requisite frequency pulses into aselected region of the MOF processing region 307, a reflector plate 314to reflect the original transmitted wave from the transducer plate 312and form standing wave through constructive interference, a MOFcontaining solution outlet 310, which like the inlet includes a fluidpump 317 to pump processed MOF containing solution into a collectionflask 315. The MOF separation and activation vessel 301 is held in anice bath 320, as best shown in FIG. 10(b).

In the process trial, a MOF containing solution with an initialtemperature of 15° C. fed from feed flask 302 into the processing region307 of the vessel 301. The transducer plate 312 was then operated toinitially pre-sonicate the MOF containing solution for 10 min (100%nominal power with the 1 MHz transducer) prior to beginning flowoperation, resulting in an increased temperature of the MOF containingsolution of 20-25° C. After this pre-sonication step, flow was initiatedby turning on the pumps 316, 317 and opening a drain valve 315A locatedat the bottom of the vessel 301. The input of MOF containing solution,along with an ice cooling bath 320 located on the sides of theprocessing region 307, enabled temperature to be maintained within anoptimal range for efficient MOF separation and activation of between 20and 40° C. across the entire process duration. The processed MOF wascollected by removing the solution from the drain valve 315.

Those skilled in the art will appreciate that the invention describedherein is susceptible to variations and modifications other than thosespecifically described. It is understood that the invention includes allsuch variations and modifications which fall within the spirit and scopeof the present invention.

Where the terms “comprise”, “comprises”, “comprised” or “comprising” areused in this specification (including the claims) they are to beinterpreted as specifying the presence of the stated features, integers,steps or components, but not precluding the presence of one or moreother feature, integer, step, component or group thereof.

The invention claimed is:
 1. A metal organic framework separationapparatus that separates a metal organic framework (MOF) from a MOFcontaining solution, comprising: a housing having a reservoir thatreceives the MOF containing solution; a high frequency ultrasoundtransducer operatively connected to the reservoir that applies megasonicfrequencies of at least 20 kHz to the MOF containing solution; and anacoustic reflector surface spaced apart from the transducer within thehousing; wherein the transducer, in use, being operated to reflect saidapplied high frequency ultrasound off the acoustic reflector surface,said acoustic reflector surface being spaced away from the highfrequency ultrasound transducer such that a standing wave is formed bythe superimposition of a reflected sound wave to form pressure nodes andantinodes where particles are collected, wherein the standing wavecauses the MOF content of the MOF containing solution to separate outfrom the MOF containing solution as an aggregated sediment.
 2. Theapparatus according to claim 1, wherein the applied high frequencyultrasound is 20 kHz to 4 MHz.
 3. The apparatus according to claim 2,wherein the high frequency ultrasound transducer comprises a platetransducer.
 4. The apparatus according to claim 1, wherein the housingcomprises a container including at least one wall position to contactthe MOF containing solution, and the transducer is high frequencyultrasound transducer is position within the reservoir or in engagementwith the at least one wall.
 5. The apparatus according to claim 1,wherein the acoustic reflector surface is generally located in front ofthe transducer, and spaced apart from that transducer.
 6. The apparatusaccording to claim 1, wherein the housing includes at least onereflector surface that reflects the applied megasonic frequencies withinthe reservoir.